During the fabrication of semi-conductor chips, organic materials, such as photo-resist commonly are used to coat the surface of the chip. The process of patterning a silicon wafer or other microelectronic substrate involves coating the substrate with photo-resist, and shining UV radiation through a mask that defines the pattern of interest onto the wafer. In the case of positive photo-resist, the portion of the photo-resist that was exposed to the radiation through the mask is then stripped away in a solution that dissolves the exposed photo-resist while not significantly affecting the unexposed photo-resist. With the mask shape so etched in the photo-resist material, the particular material to be implanted, etched or deposited on the wafer in the pattern dictated by the mask can be so implanted, etched or deposited.
After the material is on the wafer, the remaining photo-resist is removed in a different solution that is effective for stripping unexposed photo-resist. Many such solutions for stripping exposed photo-resist off of a wafer are known, including acetone and isopropyl alcohol.
Throughout an entire fabrication process for a semiconductor chip, photo-resist may be deposited and stripped a way many times using many different masks. As a wafer moves through the fabrication process, having more and more of its features embedded therein (e.g., dopants, metallizations, bond pads, protective coatings, etc.), the solution used for washing away progressive photo-resist layers must be more and more carefully selected. Particularly, as more materials become part of the chip, the photo-resist stripping solution must be carefully selected so as not to adversely affect any of the materials which have been incorporated into the chip.
For instance, in a silicon optical bench sub-assembly chip (SiOB chip) such as illustrated in FIG. 1, near the end of the fabrication process, the die contains a number of exposed materials. For instance, FIG. 1 illustrates the exposed materials on a particular SiOB die manufactured by Lucent Technologies, Inc. (see e.g., U.S. patent application Ser. No. 08/764,960 filed Dec. 22, 1995). This die is the optical bench die used in the Laser 2000 series of products offered by Lucent Technologies, Inc. of Murray Hill, N.J., the assignee of the present invention. As shown, the silicon is coated with, at least, (1) aluminum, (2) a sandwich of titanium, platinum and gold (Ti/Pt/Au), and (3) silicon dioxide (SiO.sub.2).
At the end of wafer production, the wafer is separated (or diced) into the separate dies. In the case of the die for the aforementioned SiOB chip of Lucent Technologies as well as many other dies, a final coating of photo-resist is applied over the entire wafer (with no mask) in order to protect it during the sawing operation. In particular, a common way of dicing a wafer is to cut it into the individual dies with a water cooled, diamond tipped, precision saw. This final layer of photo-resist is applied without mask so that it completely coats the wafer. The purpose of this final layer of photo-resist is to protect the surface of the dies from the water and/or silicon dust which will be thrown up during the sawing operation.
After the sawing operation, the photo-resist must be removed from the individual dies completely and without harming any of the components and features on the die.
Residual photo-resist that was not completely removed from the die can lead to several problems. One of the most notable is that residual photo-resist can lead to leakage currents or short circuits between electrical contact paths on the surface of the substrate which should be electrically isolated from each other. Residual photo-resist is particularly problematic when it is subjected to heat and becomes charred (carbonized). When carbonized, the electrical impedance of the photo-resist decreases, thus increasing the severity of leakage currents and shorts. Accordingly, residual photo-resist can substantially reduce chip yields.
Many chips are fabricated for opto-electronic applications. As the name implies, optical bench dies are basic building blocks of such opto-electronic chips. Optical bench dies are fabricated with various features to allow them to have multiple potential uses in the optical or opto-electronic applications depending on the components mounted thereon. For instance, an optical bench die may include a micro-machined cavity for placement of a micro-lens for focusing laser light. It also may be provided with mounting pads for mounting a laser diode and/or a photo detector. One such exemplary silicon optical bench die is the die use in the Laser 2000 series of products offered by Lucent Technologies, Inc., the assignee of the present application. That die, for example, has a micro-machined cavity designed to accept a micro-lens for focusing the light of a laser diode which can be mounted on a particular mounting pad on the surface of the die. The lens is bonded to an exposed aluminum layer deposited on the chip in the lens cavity. The lens itself may be made of several possible materials. One example of such material is Spinnel.TM.. The lens is bonded to the aluminum layer via a silicon dioxide coating on the lens in a process that is commonly termed AlO (aluminum oxide) bonding. Particularly, during fabrication, a chemical reaction is caused to occur that bonds the silicon dioxide layer on the lens to the aluminum on the die thus fixing the lens in the cavity (see e.g., Coucoulas, et al., 43rd ECTC Proceeding, 471-481 (1993).
During fabrication of the lens itself, various contaminants may come to be attached to the surface of the silicon dioxide layer on the lens. Such contaminants may include organic polymers such as photo-resist, oils, carbon particles, and airborne particles, among other things. While contaminants during fabrication of micro-electronic or micro-optical devices are always undesirable, they are particularly troublesome with respect to micro-lenses. Specifically, any contaminant on the surface of the silicon dioxide of the lens will reduce the efficacy of the bonding of the lens to the aluminum layer in the cavity. Accordingly, it is desirable to wash the lens just prior to bonding on the cavity in a solution which will remove the potential contaminants without affecting the silicon dioxide layer on the lens. Similarly, contaminants on the aluminum surface will also reduce the efficacy of the lens to aluminum bond.
Therefore, it is an object of the present invention to provide a safe and efficient method for removing the photo-resist from semi-conductors.
It is another object of the present invention to provide a safe and efficient method of cleaning micro-optical lenses.